(a) Field of the Invention
The present invention relates generally to a method and apparatus for testing solar cells, and more particularly, to a method and apparatus for testing the responsiveness of solar cells to a precise spectral bandwidth of light.
(b) Description of Related Art
Multiple junction tandem solar cells comprise a plurality of (typically three) distinct layers of photovoltaic devices that are electrically connected in series to one another. Each layer uses a different portion of the solar spectrum, thereby taking advantage of the fact that devices sensitive to short wavelength light can be transparent to longer wavelengths.
When solar cells are manufactured, especially for space-based applications, testing of each cell is important in order to ensure adequate performance. Multiple junction solar cells are typically tested under a steady state solar simulator using relatively slow curve tracing and data acquisition equipment. The steady state solar simulator is a large, expensive device and at best has temporal instability ("flicker") in the range of a few percent. Adjusting the spectral filtering for multiple junction solar cells requires expensive additional equipment. Inserting special filters into the steady state solar simulator in order to obtain curves depicting current as a function of voltage (I-V curves) of individual junctions is a slow process.
For space-based applications, it is desirable to simulate sunlight that impinges an orbiting satellite, which is commonly known as Air Mass Zero sunlight, or AM0. Although the light source used for simulation of AM0 sunlight for electrical tests of solar cells need not be an exact match at all wavelengths (which would be extremely difficult), it must produce the same effect on each individual junction as would AM0 sunlight.
The slow testing method using the steady state solar simulator requires each solar cell to rest on a thermally controlled block to maintain an even temperature during testing. The current testing cycle is typically thirty seconds or more per cell.
More recently, pulse simulators have been developed. One exemplary pulse simulator that is commercially available is the Large Area Pulsed Solar Simulator II, available from Spectrolab, Inc. 12500 Gladstone Avenue, Slymar, Calif. 91342, a division of Hughes Electronics Company, the assignee of the present invention. Pulse simulators use a pulse of light from a flashlamp, such as a Xenon flashlamp, rather than a steady-state source of light, in order to test solar cells. Most photovoltaic devices, particularly for aerospace use, are relatively fast devices, with time constants on the order of tens of microseconds or less. This makes the use of a short light pulse for testing solar cells an attractive solution for a number of reasons. Thermal control of the device being tested is simplified, as the pulsing of the lamp does not result in a significant increase in the temperature of the solar cells. Also, exposure of workers to harmful ultraviolet light is minimized, and the optics design to obtain uniform light at a target plane is simple and inexpensive. However, pulse simulators that use a single flashlamp still require careful tailoring of the spectrum of light emitted by the flashlamp in order to simulate sunlight.
Accordingly, there is a need for a method and system for testing solar cells that is configured so as to minimize or eliminate the aforementioned problems.